E3 ubiquitin ligases (of which over 600 are known in humans)1 confer substrate specificity for ubiquitination and are more attractive therapeutic targets than general proteasome inhibitors3,4 due to their specificity for certain protein substrates. Although the development of ligands of E3 ligase has proven challenging, in part due to the fact that they must disrupt protein-protein interactions5 recent developments have provided specific ligands which bind to these ligases. Protein-protein interaction interactions are notoriously difficult to target using small molecules due to their large contact surfaces and the shallow grooves or flat interfaces involved. Conversely, most small molecule drugs bind enzymes or receptors in tight and well-defined pockets.6 Since the discovery of nutlins, the first small molecule E3 ligase inhibitors,7 additional compounds have been reported that target Inhibitors of Apoptosis Proteins (IAPs),8,9 SCFMet30,10 and SCFCdc4;11 however, the field remains underdeveloped.
One E3 ligase with exciting therapeutic potential is the von Hippel-Lindau (VHL) tumor suppressor, the substrate recognition subunit of the E3 ligase complex VCB, which also consists of elongins B and C, Cul2 and Rbx1.12 The primary substrate of VHL is Hypoxia Inducible Factor 1α (HIF-1α), a transcription factor that upregulates genes such as the pro-angiogenic growth factor VEGF and the red blood cell inducing cytokine erythropoietin in response to low oxygen levels. While HIF-1α is constitutively expressed, its intracellular levels are kept very low under normoxic conditions via its hydroxylation by prolyl hydroxylase domain (PHD) proteins and subsequent VHL-mediated ubiquitination (FIG. 1).
Using rational design, we have generated the first small molecule ligands of Von Hippel Lindau (VHL), the substrate recognition subunit of the E3 ligase VCB, an important target in cancer, chronic anemia and ischemia.2 We have also obtained crystal structures of VHL with our most potent ligand, 15, confirming that the compound mimics the binding mode of the transcription factor HIF-1α, the major substrate of VHL.
From earlier biochemical and structural studies of a hydroxylated HIF peptide bound to VHL, it became clear that hydroxyproline played an important role in mediating this protein:protein interaction. As a consequence of that work, the present inventors developed a hydroxylated HIF peptide:VHL fluorescence polarization (FP) binding assay with which they assayed >120 compounds possessing the central hydroxyproline residue flanked by non-peptidic moieties. Further to that research, the inventors now have developed co-crystal structures of VHL complexed with seven of the top compounds. Analysis of these ligand bound structures is driving the design/synthesis of the next generation of VHL ligands, which are linked with protein binding moieties to produce bifunctional compounds according to the present invention.
A principal rationale for the present invention is the need for a small molecule E3 ligase ligand for our PROTAC (Proteolysis Targeting Chimera) technology. This technology brings targeted proteins/polypeptides to E3 ligases for ubiquitination and subsequent proteasomal degradation. In several proof-of-concept experiments, the present inventors demonstrated the utility of this approach using the short peptide sequence from HIF that binds VHL. In order to make a more ‘drug-like’ PROTAC, the inventors have replaced the HIF peptide with a ‘small molecule’ VHL ligand, thus providing a means to recruit proteins to E3 ligases for ubiquitination and degradation, to the endpoint of providing therapies based upon this protein degradation.